It could be that some part of your aftermarket shifter is hitting the guibo or driveshaft. Did this problem start just after you put in the shifter?

Hard acceleration in 1st is when the driveshaft transmits the most torque, thus the most twisting of the transmission and motor mounts occurs then. Guibo failure will show up under hard acceleration in 1st before it is noticable in other gears. However, you'd notice the whole car shaking, and you said it's only in the shifter.

Get under the car and note where all the shifter parts are in each of the gears using an inspection mirror.

Two possible solutions would be to install a different shifter, or install poly motor and trans mounts. In my E30, poly trans mounts make the cabin very loud.